Renaming vec_step in tree-vect-loop.c (to fix build on powerpc/clang)

[Gerald Pfeifer]

I have seen an increasing number of reports of GCC failing to
build with clang on powerpc (on FreeBSD, though that's probably
immaterial).

Turns out that clang has vec_step as a reserved word on powerpc
with AltiVec.

We OTOH use vec_step s as a variable name in gcc/tree-vect-loop.c.


The best approach I can see is to rename vec_step.  Before I prepare
a patch: what alternate name/spelling would you prefer?

Thanks,
Gerald